The third chapter of the Cambridge Companion to the Stoics, penned by R.J. Hankinson, is a brief introductory guide to the topic of Stoic epistemology, their theory of knowledge (which, remember, was part of logic, one of the pillars of the Stoic philosophical system, necessary to make progress in the focal area of ethics).

While the Stoic Sage never fails to see the truth, this was only an unattainable ideal, so one needs to delve into what the Stoics had to say about the rest of us in terms of access to truth and likelihood of error.

For the Stoics, there are two types of impressions, cataleptic and non-cataleptic, only the first one of which guarantees truth. This is a pretty bold affirmation, by the much more modest standards of modern epistemology, but at the least isn’t as bad as the Epicureans, who apparently thought that all perceptions are true!

Hankinson tells us that the Stoics, taking a cue from Aristotle (as opposed to Plato), adopted an empiricist view of concept formation: “Concepts are not acquired by some rational process of inductive inference; rather, they are simply built up in the soul by a suitable accretion of perceptual impressions.” No knowledge comes just from what Hume called “relations of ideas,” one always needs empirical input.

Indeed, he compares the approach to that of the philosopher David Hume: “Sextus ascribes to [the Stoics] the following soundly empiricist slogan: ‘every conceiving (noêsis) occurs either from perception (aisthêsis) or not without perception, that is to say either from an encounter or not without an encounter.’”

Hankinson quotes Sextus again: “knowledge is the secure and firm apprehension unalterable by reason, opinion is weak [and false] assent, while apprehension is intermediate between these, being assent to a cataleptic impression,” adding as commentary: “Assent to a cataleptic impression, or katalêpsis, is not yet knowledge, which must be more stable and structured.”

Skeptics, of course, had a field day with this, pointing out that if clarity and distinctiveness are internal characteristics of the impressions, then there is no way to be sure that there is any such thing as a cataleptic (i.e., truth-guaranteeing) impression.

The debate went on for quite a while, involving chiefly a dialogue between Stoics and Academics (i.e., Platonists), with the first major organic defense of Stoic doctrines being mounted by Chrysippus, so much so that later Stoics apparently used to say that “if Chrysippus had not existed, neither would the Stoa.”

Hanckinson summarizes the Academic position: “The Academic method of supporting [their criticism] was by example: there are myriad cases of people being deceived into thinking they are seeing one of a pair of identical twins, when in fact they are seeing the other; and no one can tell two sufficiently similar eggs apart.” But he points out that the Stoic “Leibnizian” metaphysics (wouldn’t be more proper to say that Leibniz’ metaphysics was Stoic?) asserted that it is impossible for two things to be exactly alike, so that sort of objection didn’t bite.

The Stoics were claiming that only some people, under some circumstances, can have cataleptic impressions, which saves them from the obvious skeptic counterexample (hey, sometimes our senses deceive us!). But Hanckinson correctly points out that they still owe their opponents an explanation of how one can tell whether an impression is or is not cataleptic, and experiencing a subjective feeling of certainty is just not enough, as we know of many instances when people are certain of notions that turn out to be false.

Hanckinson, however, adds something often overlooked within the context of a skeptic argument: “the fact that I can falsely suppose myself to be awake when I’m not (i.e., when I’m dreaming) does not show that I can falsely suppose myself not to be awake when I am. All [the Stoics] need is for there to be some cases where cataleptic impressions are had, are recognized as such, and assented to, and that in those cases there is no room for doubt.”

Moreover: “The Academics, in effect, must claim that no matter how ‘good’ the impression seems to be, it might still be false; the Stoics must hold that every case of a delusive impression will, on closer inspection, be found to fall short in respect of the clarity and distinctness requisite for genuine katalêpsis.” And again from Sextus: “the cataleptic impression is not unconditionally the criterion of truth, but only when there is no obstacle to it. For in this latter case, being evident and striking, takes hold of us, as they say, practically by the hair and drags us to assent.”

Interestingly, Hanckinson suggests that later Stoics — after a number of rounds with their skeptical colleagues — changed their mind to a degree, essentially incorporating a criterion of coherence: one compares a new impression with preceding ones as well as with other theoretical commitments one has accumulated; sometimes this will lead one to reject the (apparently) cataleptic impression as non-veridical. This coherence, however, refers to their theory of knowledge, because their theory of truth was definitely based on a concept of correspondence (of one’s notions with reality), just like all other Greek schools (and much modern philosophy, I might add).

Moreover, apparently a good argument can be made that the Stoics had developed a notion very similar to the modern idea of inference to the best explanation, sometimes referred to as consilience, or abduction, and invented by 19th century philosopher William Whewell (a mentor to Darwin):

“One might also here invoke the Stoic conception of demonstration as a type of inference to the best explanation, designed to lead us from phenomenal facts to their hidden explanations. The world is such that it will guide the diligent and practised inquirer from evident facts, by means of logically impeccable inferences, to the non-perceptible states of affairs that must obtain if the phenomena are to be as they are. This is the epistemology of the sêmeion endeiktikon, the indicative sign … The Stoics argued, for example, that the evident fact of sweating was enough to show that the skin was perforated with invisible pores”

And here is how Diogenes of Babylon, a Stoic contemporary of the Academic Carneades, put it, according to Philodemus: “it is sufficient, concerning these things and those which derive from experience, for us to be convinced in accordance with the reasonable, just as when we set sail in summer we are convinced we will arrive safely.”

As Hanckinson comments, the Stoic will therefore attempt to always put himself in a position to act according to the best available knowledge, accepting with equanimity when things don’t go his way.

We can even find, for instance in the syncretic but heavily Stoic Antiochus (as described by Cicero) a notion that today would be labeled evolutionary epistemology, or at the least as close as one could get to it before Darwin: “Nature … perfected the mind with its remaining requirements just as it did the body: for it adorned it with senses suited to the perception of things, requiring little or no assistance for their verification.”